Studies of aqueous interfaces and of the behavior of ions therein have been
profiting from a recent remarkable progress in surface selective spectroscopies,
as well as from developments in molecular simulations. Here, we summarize and
place in context our investigations of ions at aqueous interfaces employing
molecular dynamics simulations and electronic structure methods, performed in
close contact with experiment. For the simplest of these interfaces, i.e. the open
water surface, we demonstrate that the traditional picture of an ion-free surface is
not valid for large, soft (polarizable) ions such as the heavier halides. Both
simulations and spectroscopic measurements indicate that these ions can be
present and even enhanced at surface of water. In addition we show that the ionic
product of water exhibits a peculiar surface behavior with hydronium but not
hydroxide accumulating at the air/water and alkane/water interfaces. This result
is supported by surface-selective spectroscopic experiments and surface tension
measurements. However, it contradicts the interpretation of electrophoretic and
titration experiments in terms of strong surface adsorption of hydroxide; an issue
which is further discussed here. The applicability of the observed behavior of ions
at the water surface to investigations of their affinity for the interface between
proteins and aqueous solutions is explored. Simulations show that for alkali
cations the dominant mechanism of specific interactions with the surface of
hydrated proteins is via ion pairing with negatively charged amino acid residues
and with the backbone amide groups. As far as halide anions are concerned,
the lighter ones tend to pair with positively charged amino acid residues, while
heavier halides exhibit affinity to the amide group and to non-polar protein
patches, the latter resembling their behavior at the air/water interface. These
findings, together with results for more complex molecular ions, allow us to
formulate a local model of interactions of ions with proteins with the aim to
rationalize at the molecular level ion-specific Hofmeister effects, e.g. the
salting out of proteins.